1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell and an energy storage which is charged by the power generation current of the fuel cell, and assists the output of the fuel cell. Further, the present invention relates to a method of stopping operation of the fuel cell system. In particular, the present invention relates to a fuel cell system and a method of stopping operation of the fuel cell system in which when power generation of the fuel cell system is stopped, a scavenging process is performed for an anode system for starting operation of the fuel cell system at a low temperature, e.g., at a temperature below the freezing point.
2. Description of the Related Art
For example, a polymer electrolyte fuel cell employs a membrane electrode assembly which includes an anode (fuel electrode) and a cathode (air electrode), and a polymer electrolyte membrane interposed between the electrodes. The electrolyte membrane is an ion exchange membrane. The membrane electrode assembly is sandwiched between a pair of separators. A fuel gas flow field is formed between the anode and one of the separators, and an oxygen-containing gas flow field is formed between the cathode and the other of the separators. In use, normally, a predetermined numbers of the membrane electrode assemblies and separators are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas flow field. The fuel gas flows through the fuel gas flow field along the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the suitably humidified electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy. Further, in the fuel cell, an oxygen-containing gas such as the air is supplied to the oxygen-containing gas flow field, and the oxygen-containing gas flows along the cathode for reaction. At the cathode, hydrogen ions from the anode combine with the electrons and oxygen to produce water. The water is also retained at the anode due to the back diffusion from the cathode or high humidification of the fuel gas or the like.
If the water is excessively present at any of the electrodes, water clogging may occur. In view of the above, in the fuel cell system, a technique of the scavenging process for the anode (hereinafter also simply referred to as the “anode scavenging technique”) as disclosed in Japanese Laid-Open Patent Publication No. 2003-331893 is proposed. In the anode scavenging technique, when the operation of the fuel cell system is started or finished, an oxygen-containing gas is supplied to the anode in addition to the cathode for removing the water produced in the power generation from the membrane electrode assembly or the separators of the fuel cell.
In order to perform the anode scavenging process, an energy source other than the fuel cell is required. Thus, in the fuel cell system, an energy storage such as a capacitor or a battery is mounted for assisting the output of the fuel cell.
The energy storage is also utilized as an energy source for starting operation of the fuel cell system in addition to the anode scavenging process.
However, after the fuel cell system is stopped, if it becomes necessary to start operation of the fuel cell system at a low temperature such as a temperature below the freezing point due to the decrease in the outside air temperature, it has been found that, in some cases, it becomes difficult to restart operation of the fuel cell system at a low temperature such as a temperature below the freeing point. For example, this is caused by the decrease in the remaining electrical energy stored in the energy storage since the energy is consumed in the anode scavenging process.
Further, it has been found that the degradation of the fuel cell is less likely to occur as the anode scavenging process is conducted for a longer period of time.